Download The Neural Canal Opening in Myopia and Its Clinical and OCT

Best Free Papers
This paper was judged the BEST PAPER of GLAUCOMA-I Session
The Neural Canal Opening in Myopia and Its
Clinical and OCT Correlates
Dr. Nangia Vinay Kumar B., Dr. Priyanka Pardhi
T
he retinal nerve fibers converge to form the optic disc papillae and course
through the Bruchs membrane opening and through the surrounding
junctional sclera and the lamina cribrosa to form the optic nerve. The
space between the Bruchs membrane is the Neural canal opening.1 The
neural canal is often considered to be a surrogate of the optic disc margins.
In essence the clinical visible optic disc margins may define the edges of the
Bruchs membrane. In a previous study the authors showed the correlation
of the Bruchs membrane edges with the margins of the optic nerve in non
myopic eyes from a cross sectional randomly selected cohort of a population
based study, the Central India Spectral Domain OCT Study. Passage of
the retinal nerve fibres through the neural canal opening is an important
anatomical landmark. It is also important that the orbital CSF surrounds
the optic nerve in the area of the Bruch Membrane. Therefore any alteration
in the neural canal opening may translate into altering the environment
around the optic nerve. Since all anatomic parameters may be considered
to be important around the site of optic neve damage and also since the
Cerebrospinal Fluid Pressure is now considered important in the health
of the optic nerve2,3 and the Translamina cribrosa Pressure Difference4 has
been shown to be important in the evolution of glaucomatous damage, it
was considered important to further evaluate the Bruchs membrane related
neural canal opening. In this study we evaluated the neural canal opening
in myopes. Myopes have significant variation in the size of the optic disc
and presence of parapapillary atrophy. In addition the size of the eye ball
is much larger as borne out by the axial length measurments. It was the
purpose to determine the dimensions of the neural canal and their ocular
correlations in a clinic based group of myopes.
MATERIALS AND METHODS
It was a cross sectional study conducted at Suraj Eye Institute, Nagpur.
during May 2012-May 2013. 197 eyes of 100 myopic (47 males and 53 females)
patients were included. The mean age was 25.76+-10.53 (8-60) years. Only
those subjects with spherical equivalent on automated refractometry of
myopia -1.00 D or more (197 eyes of 100 subjects) were included. Subjects
with media opacities, pseudophakia, with other retinal findings, with
glaucoma and previous sugery were excluded.
73rd AIOC 2015, New Delhi
All patients underwent routine Ophthalmic evaluation, including
Automated refraction and assessment of visual acuity, Slit lamp
biomicroscopy, Applanation tonometry, Fundus evaluation with the 78
diopter lens and Indirect Ophthalmoscopy following pupillary dilatation
using tropicamide and phenylephrine. Axial length and keratometry
readings were determined on Intra-Ocular Lens Master-500. Colour
photography using 20 degree and 50 degree views. OCT was done using
Spectral domain OCT (SD-OCT). Horizontal and vertical line scan passing
through disc and macula were done in enhanced depth imaging mode.
Retinal Nerve fiber layer (RNFL) scans were done to assess the thickness
of the layer. Manual corrections were done in the scans if they were found
to had errors in automated assessment. Planimetric measurements for
diameters of optic disc, peripapillary atrophy and neural canal were made
using software provided with Spectralis.
RESULTS
The mean age was 25.76±10.53yrs, mean BCVA in Decimal Units was 0.77+0.27. The mean size of the neural canal at 180 degrees horizontally was
1501.33+-261.58 microns. (See Table 1)
Table 1: Ocular Parameters in the study group.
Parameters
Mean and SD
Range
Axial length (mm)
25.54±1.7622.51-35.57
IOP (mmHg)
15.07±2.698-25
Disc area (mm2)
Disc diameter Horizontal (mm)
2.38±0.711.01-6.19
1501.33±261.58587-2267
Diam of Neural Canal (horizontal) mm.
1872.24±3991347-3901
Max. Width of PPA in microns
561.28±43250-2227
PPA total number of Clock hours
6.53±3.532-12
In bivariate analyis diameter of neural canal showed a significant
correlation with diameter of the disc (p<0.001; r=0.235), with the BCVA
(P<0.001;-0.521), with increasing myopia (P<0.001; r=-0.752), increasing axial
length (P<0.001;0.652), with the presence of PPA (P<0.001; r=-0.411), with
the extent of PPA in clock hours (P<0.001; r=0.621), with maximum width
of PPA at any clock hour position (P<0.001; r=0.698), with width of PPA
temporally (P<0.001; r=0.566), staphyloma involving the macula (P<0.030;
r=0.154) and staphyloma involving the optic disc (P<0.001; r=0.513). Paired
T Test showed the neural canal diamater to be significantly larger than the
disc diameter
Multivariate analysis was performed using the neural canal dimensions as a
dependant variables and diameter of disc, BCVA, Axial length, maximum
Best Free Papers
width of PPA, staphyloma involving the optic disc and total number of PPA
clock hours on infra red imaging. (See Table 2).
Table 2: Showing Multivariate Analysis with Neural Canal
dimensions as the dependant variable
Parameters
Diameter of Disc Horizontal
P Values
95% CI
<0.001
0.665,0.937
BCVA
0.750-125.76,174.11
Axial length
0.006
PPA Maximum width <0.001
.573,.866
PPA Total no. of clock hrs <0.005
7.73,41.478
0.213
-46.708,207.33
Staphyloma involving the disc
11.54,66.798
DISCUSSION
The neural canal opening (NCO) is the space between the ends of the
Bruchs membrane on all sides of the optic disc. The NCO is significant
because the retinal nerve fibers converge on the optic disc and pass
through the NCO to form the optic nerve as we know it. While it has been
shown that the neural canal opening coincides generally in emmetropes
with the margins of the optic disc, studies showing such a correlation
in myopia were required. The neural canal diameter was found to be
significantly larger than the optic disc diameter (P <0.001). The neural
canal was also found to correlate significantly with increasing axial length
both in univariate and multivariate analysis. (P=0.001). The neural canal
correlates with the parapapillary atrophy (PA) width and circumferential
extent in multivariate analysis. The space between
the border of the
Bruchs membrane and the optic disc has been recently described as the
Gamma Zone.5 PA is also found to be associated with increasing axial
length. Further on analyzing the colour photographs which demonstrate
the PA, and correlating them to the OCT Enhanced Depth Images, it was
apparent that the neural canal opening margins in myopes with PA was
representative to a significant extent of the borders of the PA. In essence
this raises the importance of the PA which now becomes an indicator of
the neural canal opening. While in emmetropes the nerve fibers pass
through neural canal opening, in myopes the nerve fibres pass between
the inner borders of the intermediate scleral tissues (scleral flange) which lie
between the normal sclera and the lamina cribrosa. It is apparent that such
anatomic alterations in myopes would subject the optic nerve to an orbital
milieu that may be different from that found in the emmetropes. It may
not only alter the vascular dynamics but may also alter the mechanisms
of the translamina cribrosa pressure differential enabling a wider area
for pressure differences across the scleral flange, thereby increasing the
73rd AIOC 2015, New Delhi
susceptibility of the parapapillary retinal nerve fibers to the Trans Para
Papillary (scleral flange) pressure differential.
REFERENCES
1. Strouthidis, NG, Yang H, Fortune B, Downs C, and Burgoyne C. Detection of
Optic Nerve Head Neural Canal Opening within Histomorphomorphometric
and Spectral Domain Optical Coherence Tomography Data Sets. Invest.
Ophthalmol Vis Sci. 2009;50:214-23.
2. Berdahl J, Allingham R R, Johnson DH. Cerebrospinal Fluid Pressure Is
Decreased in Primary Open Angle Glaucoma. Ophthalmology 2008;115:763-8.
3. Ren R, Jonas JB, Tian G, Zhen Yi, et. al. Cerebrospinal Fluid Pressure in
Glaucoma. A prospective Study. Ophthalmology 2010;117:259-66.
4. Jonas JB, Nangia V, Wang N, Bhate K. et. Al. Trans-Lamina Cribrosa Pressure
Difference and Open Angle Glaucoma. The Central India Eye and Medical
Study. PLoS ONE 8 (12):e82284, doi: 10.1371/Journal.pone.0082284.
5. Jonas JB. Jonas S, Jonas R, Holbach L, et. al. Parapapillary Atrophy: Histological
Gamma Zone and Delta Zone.